This invention relates to control circuits for parallel connected electronic power systems and, more particularly, to a master clock generator for a parallel connected variable speed constant frequency power system.
AC electric power systems are usually connected in parallel to increase total system rating or in certain cases such as airborne power systems, to increase reliability. One well known type of aircraft electric power system is the DC link variable speed constant frequency (VSCF) system. Such systems include a plurality of power pole switching elements which synthesize an AC output by switching current from a pair of DC link conductors in a fixed switching pattern, which may be generated by a microprocessor or other digital circuit. When these systems are to be operated in parallel with each other or with another source such as a ground power cart, each of the power sources must be synchronized. Since DC link VSCF systems are clock based, a master clock signal is used to synchronize all of the channels.
No break power transfer (NBPT) is a special case application of a parallel power system which allows momentary paralleling with an external power source. For aircraft applications, the other source is typically an auxiliary power unit (APU) or a ground power unit (GPU). These units are typically constant speed synchronous type generators having a frequency tolerance of at least 5%. Therefore, to provide for no break power transfer, a master clock in the DC link VSCF system must be able to track the frequency of the external power source. An example of such a master clock system is illustrated in U.S. Pat. No. 4,707,142.
For aircraft power system applications, the power system must operate over an extended temperature range. It is therefore desirable to devise a master clock generator which exhibits relatively stable operating characteristics over this temperature range. It is also desirable to provide a master clock generator which is capable of providing a known default output clock signal in the event that a frequency reference signal provided by an external power source goes beyond specified frequency limits.